1. Field of the Invention
This invention relates to a magnetic field generator for an MRI, a method for assembling the same, and a method for assembling a magnet unit for the same. More Specifically, this invention relates to a magnetic field generator for MRI incorporating permanent magnets, a method for assembling the same, and a method for assembling a magnet unit for the same.
2. Description of the Related Art
A magnetic field generator for MRI uses permanent magnets. The magnet used in such an apparatus have to be made up of a plurality of magnet blocks. It is very difficult to place material blocks first and then magnetize each. Thus, in actual manufacturing, the magnetized blocks must be aligned on a plate yoke so that each of the magnet blocks has a same magnetic pole facing upward.
Conventionally, when placing the magnet blocks on the plate yoke, a surface of the plate yoke is first applied with adhesive, and then magnet blocks are bonded to the surface, as disclosed in the Japanese Patent No. 2,699,250 for example. According to such a bonding method, upper surfaces of respective magnet blocks bonded to the plate yoke surface are not flush with each other, making an uneven surface. A magnetic field generator incorporating the permanent magnets made of such magnet blocks is apt to produce ununiform magnetic field between a pair of piece poles opposed to each other. Further, pole pieces for correcting the ununiformity of the magnetic field may be tilted to produce ununiformity in the magnetic field. Generally, after a step of mounting a pair of permanent magnets to oppose each other, a step of adjustment for uniformly distributing the magnetic field is indispensable. However, if the magnet blocks are mounted according to the above method, the ununiformity of the magnetic field is so large that the adjustment becomes very time consuming with a lot of sub-steps.
Further, according to the above method of bonding the magnet blocks, the magnet blocks each having a very intense magnetism have to be placed from above, onto the upper surface of the plate yoke, making extremely difficult to fit each of the magnet blocks snugly to adjacent magnet blocks. More specifically, when mounting, each magnet block is held with a face of predetermined magnetic pole facing upward. When the magnet block is brought above the other magnet block which is already fixed onto the plate yoke, a pulling force is generated between the two. Further, when the two magnet blocks are brought in adjacency, a repelling force is generated between the two. Since the magnet block to be placed is under such intense forces, the magnet block must be firmly held for safety while being transported. For a conventional holding mechanism, it is very difficult to fit the magnet block snugly to the place of bonding efficiently against these strong forces.
A pair of magnet units thus assembled as above are then opposed to each other so the permanent magnets are faced at a predetermined distance. This process is achieved by first assembling one magnet unit, then connecting a column yoke to the magnet unit, and finally connecting the other magnet unit to the column yoke.
The column yoke is to magnetically connect the pair of magnet units, and therefore must be made of a magnetic material. Thus, when the column yoke is connected to the magnet unit, the column yoke is brought under the pulling force from the magnet unit, making difficult to connect the two at a high accuracy. Likewise, when the second magnetic unit is connected to the column yoke already connected to the first magnet unit, it is also difficult to connect the two at a high accuracy.